Creep rupture properties of welded joints in W-containing 9Cr ferritic heat-resistant steel

Abstract

Summary This paper describes an investigation of the creep rupture properties of welded joints produced from W-containing 9Cr-Mo-W steel. The creep rupture properties of the HAZ are also studied using simulated HAZ specimens subjected to PWHT (post-weld heat treatment). The effect of W on the creep rupture strength of the welded joints is examined. Creep rupture tests of GTA (TIG) welded joints are conducted. The longest creep rupture time is around 20 000 hours. In the creep rupture tests, the welded joints rupture in the base metal at higher applied stresses, rupturing in the low-ductility fine-grained HAZ adjacent to the base metal at lower applied stresses. When the welded joints rupture in the base metal, their creep rupture strength is as high as that of the base metal. When the welded joints rupture in the HAZ, however, their creep rupture strength is lower than that of the base metal. The cracking which occurs in the HAZ is TYPE IV cracking which tends to affect the welded joints of ferritic heat-resistant steel. TYPE IV cracking is the type which occurs in the fine-grained HAZ at a lower stress than the creep rupture strength of the base metal without being associated with any heavy deformation. In the creep rupture tests, the simulated HAZ specimens heated to a temperature around Ac1 and Ac3 give a lower creep rupture strength than that of the base metal. The simulated HAZ specimens heated to the Ac3 temperature give the lowest creep rupture strength. A comparison of the creep rupture strengths of welded joints produced from 9Cr-Mo-W steel and 9Cr-1 Mo-Nb-V (mod. 9Cr-Mo) steel suggests that W improves the creep rupture strength of both welded joints and base metal.